Cellular phone systems operate in numerous frequencies and with numerous standards globally. Typically, each frequency band supports several standards. Service providers operate in multiple bands, use different standards, and require that the handsets that they support be usable through their system. Handset manufacturers make handsets for numerous service providers and want to minimize the number of radio types they offer.
The three major standards in use are: GSM, which is worldwide; CDMA which is mostly in North America, and is evolving into WCDMA; and WCDMA which is being deployed worldwide. There are six principal bands used in Europe, much of Asia, and/or North America. They are not exclusive of one another. These bands are also typical but not exclusively used elsewhere in the world, although the great bulk of the cell band communication occurs in these. Handsets currently in production support only some subset of these bands and standards. However, advanced designs are required to satisfy as many of these bands and standards as possible.
TABLE 1NomenclatureGSM/WCDMALocationFrequenciesCellularNorth America 824 to 849 MHz TransmitBand/WCDMA V 869 to 894 MHz ReceiveE-GSM/WCDMAEurope and Asia 880 to 915 MHz Transmit900 925 to 960 MHz Receivenone/WCDMA - IVNorth America1710 to 1770 MHz Transmit2110 to 2170 MHz ReceiveDCS/WCDMA - IIIEurope and Asia1710 to 1785 MHz Transmit1805 to 1880 MHz ReceivePCS/WCDMA IINorth America1850 to 1910 MHz Transmit1930 to 1990 MHz ReceiveUMTS/WCDMA - IEurope and Asia1920 to 1980 MHz Transmit2110 to 2170 MHz ReceiveThe standards have mutually incompatible requirements. One such is the multiple bands and standards. The GSM standard allows transmit and receive to alternate (half duplex). GSM also uses frequency division, with transmit and receive portions separated by frequency. CDMA and WCDMA use only frequency division, transmitting and receiving at the same time (full duplex). WCDMA, CDMA and GSM can also coexist within any particular band. In Europe, GSM and WCDMA are offered in the DCS band, GSM, and perhaps soon WCDMA, are offered in the E-GSM band. In North America, GSM, CDMA and WCDMA are offered in both the 850 and PCS bands. There is a new WCDMA only band rolling out in North America, WCDMA IV.
The differences between the full duplex standards (CDMA and WCDMA) and the half duplex (GSM) are another constraint. These differences are such that the transmit portion of the handsets cannot be shared between half and full duplex. However, the Radio Frequency (RF) part of the receive portion of the full duplex portions could be utilized by the half duplex standards. Typically the full duplex path is more expensive and has performance higher than required for half duplex, but in a handset that supports both standards it may be economical to share.
There are numerous differences between CDMA and WCDMA. The important two are higher data rate and increased sensitivity to distortion on the part of WCDMA. The RF filters, RF power amplifiers and RF low noise amplifiers in a front end (the RF part) designed for WCDMA can also be used for CDMA. The base band portion of the handset would have different requirements.
Another constraint is that unless many more base stations are built than available today, WCDMA handsets require at least two receivers to achieve the required higher data rate. These multiple receivers require multiple antennas.
The more difficult interference specifications of WCDMA require a very non-distorting front end. Specifically, any active component between the antenna and the WCDMA power amplifiers will be a source of distortion. Switches in particular add loss and are a major source of distortion.
Another constraint is that the North America bands are not compatible with the European & Asian bands. The Tx portion of the PCS band (1850 to 1910 MHz) overlaps with the Rx portion of the DCS band (1805 to 1880 MHz). The Rx portion of the PCS band (1930 to 1990 MHz) overlaps with the Tx portion of the WCDMA-I band (1920 to 1980 MHz). The Rx portion of the Cell band (869 to 894 MHz) overlaps with the Tx portion of the E-GSM band (880 to 915 MHz). As a consequence, these interfering bands cannot connect to the same antenna unless switching is utilized.
FIG. 1 illustrates prior art architecture. This architecture supports one WCDMA band along with the GSM, and so no longer solves the problem for the market. FIG. 2 is closer to a solution, but offers limited WCDMA coverage worldwide. Additionally, there is a switch in the WCDMA-V Band. This switch is difficult and expensive to implement.